1. Field of the Invention
The present invention relates to an optical pickup apparatus for writing or reading a video signal, an audio signal, or an information signal such as a computer data signal or the like to/from an optical system recording medium such as an optical disc.
2. Description of Related Art
Research into a high density recording of not only an audio signal but also an information signal including a video signal and computer data to optical discs is ongoing. For example, when an information signal including a video signal of a movie of about two hours is recorded onto an optical disc of a size equal to that of a well-known compact disc on which only the audio signal is recorded, the recording density of the optical disc has to be set fairly high. To realize a high recording density, it is considered to decrease an interval between tracks to record the information signal. If the track interval, namely a track pitch, is reduced, there arises such a problem that a crosstalk amount which is mixed into the information signal increases and a quality of a reproduction information signal deteriorates. Particularly, when the optical disc is inclined with respect to an optical axis, a situation occurs such that the crosstalk amount from one adjacent track increases and a preferable signal reproduction cannot be performed.
With a conventional pickup apparatus, so long as the reading is performed for an information signal recorded on an optical disc in which the track pitch is set at a large value, a good reading operation is achieved. It is, however, not possible to perform an accurate reading operation of the information signal from an optical disc in which a track pitch is reduced for high recording density without being adversely affected by crosstalk. Even in cases of reproducing an optical disc with a large track pitch, if the optical disc is inclined with respect to the optical axis as mentioned above, then a crosstalk amount from one adjacent track increases. The conventional pickup apparatus, therefore, has a disc tilt sensor for detecting a tilt of the disc and adjusting the optical axis to the optical disc in accordance with a tilt detection amount detected by the disc tilt sensor.
FIG. 1 shows a disc tilt sensor which is used in the conventional pickup apparatus. In this disc tilt sensor, a light emitting diode 11 generates a light beam for detecting a disc tilt and directs it to a disc 12. Photodetectors 13 and 14 are provided in the disc radial direction so as to sandwich the light emitting diode 11 and receive a reflection light from the disc 12, respectively. Light receiving level signals of the photodetectors 13 and 14 are supplied to a differential amplifier 15 and a difference between them is extracted. For example, when the right side of the disc 12 is tilted downwards, an output level of the photodetector 13 increases and an output level of the photodetector 14 decreases, so that the tilt of the disc can be detected from the level difference. An output signal of the differential amplifier 15 is supplied to a driving mechanism for adjusting the tilt of the whole pickup and the tilt of the whole pickup is automatically adjusted so as to correct the tilt detected by the disc tilt sensor.
However, since the provision of the tilt sensor and driving mechanism leads to an enlarged size of the pickup apparatus, which in turn causes the pickup itself to become unable to move at a high speed. There, consequently, is such a drawback that it is difficult to access to a target track at a high speed. Since it is necessary to accurately adjust an angle of mounting of the tilt sensor, a further drawback results that it is troublesome to perform such an adjustment at an assembling stage of the apparatus. Further, although the crosstalks from the adjacent tracks can be balanced by an output of the tilt sensor, since a crosstalk amount itself cannot be measured by the tilt sensor, a track interval cannot be reduced significantly. On the other hand, there has been proposed a method of previously recording marks to measure the crosstalk onto the tracks. However, since the marks use a predetermined capacity in a disc recording capacity irrespective of information to be recorded by the user, there is a drawback such that the capacity in which information can be recorded onto the disc is reduced.